To drive P.D.C. drill bits it is known to use a motor located on the drill string which is driven by the drilling mud that is pumped through the drill string.
Turbine motors have been used for this purpose though they operate at high speed producing a relatively low torque whereas P.D.C. requires a low speed drive at a relatively high torque.
Mono type positive displacement motors (P.D.M.s) are also used for this purpose and offer the advantages of ease of downhole monitoring and simplicity of operation. However, this form of motor relies upon the use of elastomer material in the stator which limits the operational temperature and pressure thus lowering the total output and efficiency of the motors which are also relatively expensive to manufacture and maintain.
Motors which operate in a similar manner to roller vane type pumps have not been used previously in this application, despite the simplicity and robustness of the design. The reason for this is that motors of this type require a relatively large diameter in relation to power output.
A reduction in roller size would reduce the diameter of such a motor but this would also lead to a large reduction in the volumetric efficiency and the greater fluid velocity would lead to increased erosion of the motor. In addition, as roller vane type machines rely heavily on centrifugal force to hold the rollers in sealing contact with the inner wall of the stator, motors of this type would be unable to start from rest unless specific means were included, such as, for example springs, to urge the vanes radially outwards.
In all hydraulic vane motors/pumps the pumping effect provided behind the rollers/blade is a major disadvantage. In the case of this invention, where part of the roller effectively creates a part of the internal bore of the rotor, this pumping effect is eliminated and the hydraulic forces on the roller become an advantage rather than a disadvantage.